U.S. patent application number 11/908868 was filed with the patent office on 2009-05-21 for production method for electric double layer capacitor.
Invention is credited to Kotaro Kobayashi, Kazuhiro Minami.
Application Number | 20090126172 11/908868 |
Document ID | / |
Family ID | 36991854 |
Filed Date | 2009-05-21 |
United States Patent
Application |
20090126172 |
Kind Code |
A1 |
Kobayashi; Kotaro ; et
al. |
May 21, 2009 |
Production Method For Electric Double Layer Capacitor
Abstract
A method for manufacturing an electric double layer capacitor
comprising polarizable electrodes formed from a carbon material
having graphite-like microcrystalline carbon, wherein said carbon
material is stored for a predetermined period of time while holding
said carbon material in contact with an electrolytic solution, and
thereafter, at a temperature higher than room temperature, said
capacitor is charged at least once with an end-of-charge voltage
higher than an expected operating voltage of said capacitor, while
applying to said electrodes, at the start of said charging, a
pressure that is necessary to keep said carbon material from
expanding in a thickness direction thereof during said
charging.
Inventors: |
Kobayashi; Kotaro; (Tokyo,
JP) ; Minami; Kazuhiro; (Tokyo, JP) |
Correspondence
Address: |
Allan M. Wheatcraft;W.L. Gore & Associates, Inc.
551 Paper Mill Road, P.O. Box 9206
Newart
NJ
19714
US
|
Family ID: |
36991854 |
Appl. No.: |
11/908868 |
Filed: |
March 17, 2006 |
PCT Filed: |
March 17, 2006 |
PCT NO: |
PCT/JP2006/305893 |
371 Date: |
December 19, 2008 |
Current U.S.
Class: |
29/25.03 |
Current CPC
Class: |
H01G 11/32 20130101;
H01G 11/86 20130101; Y02E 60/13 20130101; Y10T 29/43 20150115 |
Class at
Publication: |
29/25.03 |
International
Class: |
H01G 9/038 20060101
H01G009/038 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 18, 2005 |
JP |
2005-080463 |
Claims
1. A method for manufacturing an electric double layer capacitor
comprising polarizable electrodes formed from a carbon material
having graphite-like microcrystalline carbon, wherein said carbon
material is stored for a predetermined period of time while holding
said carbon material in contact with an electrolytic solution, and
thereafter, at a temperature higher than room temperature, said
capacitor is charged at least once with an end-of-charge voltage
higher than an expected operating voltage of said capacitor, while
applying to said electrodes, at the start of said charging, a
pressure that is necessary to keep said carbon material from
expanding in a thickness direction thereof during said
charging.
2. A method as claimed in claim 1, wherein said carbon material is
stored at a temperature higher than room temperature.
3. A method as claimed in claim 1, wherein the temperature during
said storage and the temperature during said charging are both
maintained at 35.degree. C. or higher.
4. A method as claimed in claim 1, wherein said carbon material is
stored for 12 hours or longer.
5. A method as claimed in claim 1, wherein said pressure is within
a range of 4.9.times.10.sup.5 to 4.9.times.10.sup.6 Pa.
6. A method as claimed in claim 1, wherein said end-of-charge
voltage is within a range of 110 to 135% of said expected operating
voltage.
7. A method as claimed in claim 1, wherein said carbon material
having graphite-like microcrystalline carbon has a specific surface
area not larger than 200 m.sup.2/g as measured by a BET
single-point method before said charging, and an interlayer
distance d.sub.002 lying within a range of 0.350 to 0.385 nm as
measured by an X-ray diffraction method.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method for manufacturing
an electric double layer capacitor.
BACKGROUND ART
[0002] In recent years, electric double layer capacitors capable of
charging and discharging with a large current have been attracting
attention as a type of electric power storage device for
applications requiring frequent charge/discharge cycles, for
example, auxiliary power supplies for electric vehicles, solar
cells, wind power generation, etc. There is therefore a need for an
electric double layer capacitor that has high energy density, is
capable of fast charging and discharging, and has excellent
durability.
[0003] An electric double layer capacitor comprises a pair of
polarizable electrodes as a positive electrode and a negative
electrode disposed opposite each other with a separator interposed
therebetween. Each polarizable electrode is impregnated with an
aqueous or non-aqueous electrolytic solution, and is combined with
a current collector.
[0004] A carbon material having a graphite-like microcrystalline
carbon is known for use as a polarizable electrode material for
electric double layer capacitors (Japanese Unexamined Patent
Publication No. H11-317333). This carbon material is prepared by
controlling the activation process of the raw material so that the
distance between the layers of the crystallites of the
graphite-like microcrystalline carbon lies within a range of 0.350
to 0.385 nm. A microcrystalline carbon having this specific
interlayer distance exhibits the property that when the usual
expected operating voltage (rated voltage) is applied while holding
the carbon in contact with an electrolytic solution, only a small
capacitance can be obtained because of its small specific surface
area, but once a voltage higher than the expected operating voltage
is applied, electrolyte activation occurs with electrolyte ions
inserted between the layers, thus producing a high capacitance.
Once the ions are inserted, the carbon material maintains
high-capacitance even when it is repeatedly used with the expected
operating voltage. Compared with activated carbon commonly used as
a carbon material for electric double layer capacitors, the above
carbon material has a high withstand voltage and permits energy
density to be increased significantly, and therefore has been
attracting attention as a carbon material that can replace
activated carbon.
[0005] When performing initial charging to complete the electric
double layer capacitor whose polarizable electrodes are formed from
the carbon material having graphite-like microcrystalline carbon,
it is known to use a voltage higher than the expected operating
voltage in order to forcefully insert electrolyte ions between the
layers of the microcrystalline carbon (Japanese Unexamined Patent
Publication No. 2000-077273). It is also known that, during the
initial charging of the electric double layer capacitor, a pressure
that resists the expansion of the polarizable electrodes due to the
insertion of the electrolyte ions is applied to the polarizable
electrodes in order to suppress the expansion of the polarizable
electrodes (Japanese Unexamined Patent Publication Nos. 2000-068164
and 2000-068165). Furthermore, it is known that, when performing
such initial charging, the electric double layer capacitor is
charged for a time longer than the rated charge time of the
electric double layer capacitor (Japanese Unexamined Patent
Publication No. 2000-100668). Any of these prior art techniques is
intended to improve the method of initial charging in order to
increase the capacitance of the electric double layer capacitor.
However, no prior art technique is known that focuses on the
processing of the polarizable electrodes before charging or the
temperature during the charging.
DISCLOSURE OF THE INVENTION
[0006] It is an object of the present invention to provide a method
for manufacturing an electric double layer capacitor that achieves
a high capacitance density compared with the prior art.
[0007] According to the present invention, there is provided
[0008] (1) a method for manufacturing an electric double layer
capacitor comprising polarizable electrodes formed from a carbon
material having graphite-like microcrystalline carbon, wherein the
carbon material is stored for a predetermined period of time while
holding the carbon material in contact with an electrolytic
solution, and thereafter, at a temperature higher than room
temperature, the capacitor is charged at least once with an
end-of-charge voltage higher than an expected operating voltage of
the capacitor, while applying to the electrodes, at the start of
the charging, a pressure that is necessary to keep the carbon
material from expanding in a thickness direction thereof during the
charging.
[0009] According to the present invention, there is also
provided
[0010] (2) a method as described in item (1), wherein the carbon
material is stored at a temperature higher than room
temperature.
[0011] According to the present invention, there is also
provided
[0012] (3) a method as described in item (1) or (2), wherein the
temperature during the storage and the temperature during the
charging are both maintained at 35.degree. C. or higher.
[0013] According to the present invention, there is also
provided
[0014] (4) a method as described in any one of items (1) to (3),
wherein the carbon material is stored for 12 hours or longer.
[0015] According to the present invention, there is also
provided
[0016] (5) a method as described in any one of items (1) to (4),
wherein the pressure is within a range of 4.9.times.10.sup.5 to
4.9.times.10.sup.6 Pa.
[0017] According to the present invention, there is also
provided
[0018] (6) a method as described in any one of items (1) to (5),
wherein the end-of-charge voltage is within a range of 110 to 135%
of the expected operating voltage.
[0019] According to the present invention, there is also
provided
[0020] (7) a method as described in any one of items (1) to (6),
wherein the carbon material having graphite-like microcrystalline
carbon has a specific surface area not larger than 200 m.sup.2/g as
measured by a BET single-point method before the charging, and an
interlayer distance d.sub.002 lying within a range of 0.350 to
0.385 nm as measured by an X-ray diffraction method.
BEST MODE FOR CARRYING OUT THE INVENTION
[0021] A method for manufacturing an electric double layer
capacitor according to the present invention is characterized in
that a carbon material having graphite-like microcrystalline carbon
(hereinafter referred to as "graphite-like carbon material") is
stored for a predetermined period of time while holding the carbon
material in contact with an electrolytic solution, and thereafter,
at a temperature higher than room temperature, the capacitor is
charged at least once with an end-of-charge voltage higher than the
expected operating voltage of the capacitor, while applying to the
electrodes, at the start of the charging, a pressure that is
necessary to keep the carbon material from expanding in its
thickness direction during the charging.
[0022] The graphite-like carbon material used in the manufacturing
method of the electric double layer capacitor according to the
present invention has microcrystalline carbon. Generally, the above
carbon material is not suitable for use as an electrode material
for electric double layer capacitors because its specific surface
area is small. However, when the distance d.sub.002 between the
layers of the microcrystalline carbon (measured by an X-ray
diffraction method) lies within a specific range, i.e., the range
of 0.350 to 0.385 nm, this graphite-like carbon material exhibits a
high capacitance as a polarizable electrode despite its small
specific surface area. More preferably, the interlayer distance
d.sub.002 is in the range of 0.360 to 0.380 nm, because then the
expression of capacitance due to the interlayer insertion of
electrolyte ions becomes more pronounced. If the interlayer
distance d.sub.002 is smaller than 0.350 nm, the interlayer
insertion of electrolyte ions becomes difficult to occur, and the
capacitance therefore decreases. Conversely, if the interlayer
distance d.sub.002 is larger than 0.385 nm, the interlayer
insertion of electrolyte ions likewise becomes difficult to occur,
and the capacitance decreases, which is not desirable.
[0023] The specific surface area of this graphite-like carbon
material is preferably 200 m.sup.2/g or less, more preferably 100
m.sup.2/g or less, and particularly preferably 20 m.sup.2/g or
less. If the specific surface area is larger than 200 m.sup.2/g, a
sufficient capacitance density can be obtained without relying on
the method of the present invention. However, the amount of
functional groups existing on the surface of the graphite-like
carbon material increases, and when a voltage is applied, these
functional groups decompose, significantly degrading the
performance of the electric double layer capacitor. The values of
the specific surface area given here were obtained by a BET
single-point method (drying temperature: 180.degree. C., drying
time: 1 hour) by using MONOSORB manufactured by Yuasa Ionics Co.,
Ltd.
[0024] A low-temperature calcined carbon material that is not well
activated can be used as the graphite-like carbon material, and can
be produced using various kinds of materials such as wood, fruit
shells, coal, pitch, coke, etc., that are commonly used as
materials for activated carbon. For example, the material can be
produced by heat treating it in an inert atmosphere before
activation thereby preventing activation from proceeding
substantially, or by activating it only briefly. As for the heat
treatment, it is preferable to calcine the material at relatively
low temperatures of about 600 to 1000.degree. C. For other
graphite-like carbon materials advantageous for use in the present
invention and the production methods thereof, refer to Japanese
Unexamined Patent Publication Nos. H11-317333, 2000-077273,
2000-068164, 2000-068165, and 2000-100668. The graphite-like carbon
material is contained in each polarizable electrode in an amount
ranging from 50% to 99% by mass, or more preferably from 65% to 85%
by mass, with respect to the combined mass of the graphite-like
carbon material and the binder and conductive agent described
hereinafter. If the content of the graphite-like carbon material is
smaller than 50% by mass, the energy density of the electrode
decreases. Conversely, if the content exceeds 99% by mass, the
amount of the binder becomes insufficient, resulting in an
inability to form a continuous sheet-like electrode.
[0025] The electrode for the electric double layer capacitor
contains a conductive agent for conferring electrical conductivity
to the graphite-like carbon material. Carbon black such as Ketjen
black or acetylene black, nanocarbon such as fullerene, carbon
nanotube, or carbon nanohorn, or powdered graphite or the like can
be used as the conductive agent. The conductive agent should be
added preferably in an amount ranging from 1% to 40% by mass, or
more preferably in an amount ranging from 3% to 20% by mass, with
respect to the combined mass of the conductive agent, the
graphite-like carbon material, and the binder. If the amount of the
conductive agent added is smaller than 1% by mass, the internal
resistance of the electric double layer capacitor increases.
Conversely, if the amount added exceeds 40% by mass, the energy
density of the electrode decreases.
[0026] The electrode for the electric double layer capacitor
contains a binder for binding the conductive agent to the
graphite-like carbon material to form a sheet-like structure. A
known material such as polytetrafluoroethylene (PTFE),
polyvinylidene fluoride (PVDF), polyethylene (PE), polypropylene
(PP), styrene-butadiene rubber (SBR), acrylonitrile-butadiene
rubber (NBR), etc., can be used as the binder. The binder should be
added preferably in an amount ranging from 1% to 30% by mass, or
more preferably in an amount ranging from 3% to 20% by mass, with
respect to the combined mass of the binder, the graphite-like
carbon material, and the conductive agent. If the amount of the
binder added is smaller than 1% by mass, a continuous sheet-like
electrode cannot be formed. Conversely, if the amount added exceeds
30% by mass, the internal resistance of the electric double layer
capacitor increases.
[0027] The electrode for the electric double layer capacitor here
can be manufactured by a method similar to the conventional method
that uses activated carbon. For example, after the particle size of
the graphite-like carbon material prepared by the earlier described
method has been adjusted so that the mean particle size D50 falls
within the range of about 5 to 200 .mu.m, the conductive agent and
the binder are added to the carbon material, and the mixture is
kneaded and rolled into a sheet-like form. When kneading, various
liquid agents such as water, ethanol, acetonitrile, siloxane, etc.
may be used singly or mixed together in a suitable combination. The
electrode thickness of the electric double layer capacitor is
preferably 50 to 500 .mu.m, and more preferably 60 to 300 .mu.m. If
the thickness is smaller than 50 .mu.m, pinholes are likely to be
formed in the electrode. Conversely, if the thickness exceeds 500
.mu.m, the density of the electrode cannot be increased, and as a
result, the energy density of the electrode decreases. The values
of the electrode thickness given here were obtained by measuring it
using a dial thickness gauge "SM-528" manufactured by Teclock Co.,
Ltd., without applying any load other than the instrument's spring
load.
[0028] By combining the thus produced sheet-like electrodes with a
suitable separator and current collectors commonly used in an
electric double layer capacitor, and by impregnating the electrodes
with a suitable electrolytic solution, the electric double layer
capacitor according to the present invention can be fabricated.
Various kinds of sheet materials, including a metallic sheet of
aluminum, titanium, stainless steel, or the like, and a
non-metallic sheet such as a conductive polymer film, a
conductive-filler-containing plastic film, or the like, can be used
for the current collectors. The sheet-like current collectors may
each be formed so as to contain pores in a portion thereof or over
the entire surface thereof. The sheet-like electrode and the
sheet-like current collector can function as the electrode and
current collector by simply pressure-bonding one to the other.
However, in order to reduce the contact resistance between them,
they may be bonded together by using a conductive paint as a
bonding material, or by applying a conductive paint over the
electrode or the current collector and pressure-bonding them after
drying. An insulating material, such as microporous paper or glass
or a porous plastic film of polyethylene, polypropylene,
polytetrafluoroethylene, or the like, can be used for the
separator. The separator thickness is generally in the range of
about 10 to 100 .mu.m. A liquid electrolyte or an electrolytic
solution prepared by dissolving an electrolyte in an organic
solvent may be used as the electrolytic solution. A person skilled
in the art can select a suitable one according to the purpose.
[0029] According to the present invention, prior to the initial
charging of the thus fabricated electric double layer capacitor,
the graphite-like carbon material is stored for a predetermined
period of time while holding the carbon material in contact with
the electrolytic solution. By providing the storing step before the
initial charging step, the electrolytic solution can be made to
sufficiently penetrate into the pores of the graphite-like carbon
material before the charging, compared with the case where such a
storing step is not provided, as a result, the capacitance per unit
volume (capacitance density) of the resulting electric double layer
capacitor significantly increases.
[0030] The carbon material may be stored at room temperature, or in
other words about 25.degree. C. Preferably, the material should be
stored at a temperature higher than room temperature, for example,
at 35.degree. C. or higher. When the storage temperature is
35.degree. C. or higher, the viscosity of the electrolytic solution
decreases, making it easier for the electrolytic solution to
penetrate into the pores of the graphite-like carbon material
before the charging. Conversely, when the storage temperature is
lower than room temperature, the viscosity of the electrolytic
solution increases, making it difficult for the electrolytic
solution to penetrate into the pores, therefore, it is preferable
to store the material at a temperature not lower than 20.degree. C.
There is no specific upper limit to the storage temperature, but it
is preferable to store the material at a temperature that does not
cause the electrolytic solution to decompose. For example, when a
propylene carbonate solution of triethylmethylammonium
tetrafluoroborate is used as the electrolytic solution, the storage
temperature should be held lower than 80.degree. C. because the
electrolytic solution decomposes at temperatures of 80.degree. C.
or higher.
[0031] The storage time differs depending on the storage
temperature, the temperature during charging, and other process
parameters, but when storing the material at room temperature, the
storage time is generally 12 hours or longer, preferably 18 hours
or longer, or more preferably 24 hours or longer. When storing the
material at room temperature, if the storage time is shorter than
12 hours, the electrolytic solution cannot sufficiently penetrate
into the pores of the graphite-like carbon material before
charging. On the other hand, when storing the material at a
temperature higher than room temperature, even if the storage time
is significantly shorter than 12 hours, the intended effect of the
present invention may be achieved. There is no specific upper limit
to the storage time, but if the material is stored for an extended
period of time, a further increase in the capacitance density
cannot be expected and the manufacturing efficiency of the electric
double layer capacitor drops.
[0032] According to the present invention, after the storing step,
the capacitor is charged while applying to the electrodes, at the
start of the charging, a pressure that is necessary to keep the
graphite-like carbon material from expanding in its thickness
direction during the charging. The graphite-like carbon material
has the characteristic that when it is used as an electrode
material for an electric double layer capacitor, it exhibits a high
capacitance, but expands when a voltage is applied (for charging).
In other words, when the electric double layer capacitor is
fabricated by forming the graphite-like carbon material in a
sheet-like shape and laminating it to one or both sides of the
current collector, and a voltage is applied between the current
collectors, the graphite-like carbon material expands predominantly
in the voltage application direction due to the current collectors.
Since the volume of the electric double layer capacitor increases
as the graphite-like carbon material used for the electrodes
expands, the capacitance per unit volume (capacitance density) of
the electric double layer capacitor decreases correspondingly even
if the capacitance of the electric double layer capacitor
increases. Accordingly, if the increase of the capacitance is to
provide a practical benefit, it is preferable to minimize the
increase of the volume of the electric double layer capacitor due
to the expansion of the graphite-like carbon material. Therefore, a
pressure that resists the pressure (expansion pressure) produced by
the expansion of the graphite-like carbon material is applied
externally to the electrodes in order to suppress the increase in
the volume of the electric double layer capacitor. For example, by
externally applying a pressure of about 4.9.times.10.sup.5 to
4.9.times.10.sup.6 Pa to the electrodes during the charging, the
capacitance can be effectively increased. Here, if the expansion of
the electrode volume is completely suppressed, the capacitance
produced between the electrodes is almost the same as when a free
expansion is permitted.
[0033] According to the present invention, by charging the
capacitor at a temperature higher than room temperature, the
capacitance density of the resulting electric double layer
capacitor significantly increases. The capacitor is charged
preferably at 35.degree. C. or higher, or more preferably at
40.degree. C. or higher. If the temperature during the charging is
lower than room temperature, the viscosity of the electrolytic
solution increases, making it difficult for the electrolytic
solution to penetrate into the pores that are newly formed in the
graphite-like carbon material by the activation of the electrolyte
during the charging. There is no specific upper limit to the
temperature during the charging, but as in the case of storage, it
is preferable to charge the capacitor at a temperature that does
not cause the electrolytic solution to decompose.
[0034] The charging is done with an end of charge voltage higher
than the expected operating voltage of the electric double layer
capacitor. The electrolyte ions (together with the solvent in the
case of an organic solvent) are thus inserted between the layers of
the graphite-like carbon material, and thereafter, an electric
double layer is formed. It is preferable to set the end-of-charge
voltage within the range of 110% to 135% of the expected operating
voltage. If the end-of-charge voltage is lower than 110% of the
expected operating voltage, a high capacitance density cannot be
achieved because the electric activated is not done sufficiently.
If it exceeds 135%, the decomposition of the electrolytic solution
is promoted, and as a result, the performance of the electric
double layer capacitor significantly drops.
EXAMPLES
[0035] Examples of the present invention will be described in
detail below.
Example 1
[0036] A pitch-based carbon precursor was calcined at 800.degree.
C. in an inert atmosphere to carbonize it, and potassium hydroxide
whose weight ratio to the carbonized material was 2 was mixed with
it. Then, the mixture was activated through heat treatment at
700.degree. C. in an inert atmosphere, to produce a graphite-like
carbon material having a BET specific surface area of 10 m.sup.2/g.
When this graphite-like carbon material was analyzed by X-ray
diffraction, the distance d.sub.002 between the layers of the
microcrystalline carbon was 0.360 nm. Ethanol was added to a
mixture consisting of 80% by mass of the graphite-like carbon
material, 10% by mass of Ketjen black powder as a conductive agent
("EC600JD" manufactured by Ketjen Black International Co., Ltd),
and 10% by mass of polytetrafluoroethylene powder as a binder
("TEFLON (registered trademark) 6J" manufactured by Mitsui DuPont
Fluorochemicals Co., Ltd.), and the resulting mixture was kneaded
and then rolled three times to produce a polarizable sheet of a
width of 100 mm and a thickness of 200 .mu.m. A highly pure etched
aluminum foil of a width of 150 mm and a thickness of 50 .mu.m
("C512" manufactured by KDK Corporation) was used as a current
collector, and a conductive adhesive liquid ("GA-37" manufactured
by Hitachi Powdered Metals Co., Ltd.) was applied in an amount of
30 g/m.sup.2 on both sides of the current collector by using a
coating roller. In terms of dry mass, the amount of the applied
liquid was 6 g/m.sup.2. After the application of the liquid, a long
length of the polarizable sheet was placed over each of the
liquid-applied portions (on both sides) of the current collector,
and the sheet and the current collector were bonded together by
passing them through compression rolls, thus obtaining a laminated
sheet with the contacting faces reliably bonded together. Then, the
laminated sheet was passed through a continuous hot air drying
machine whose temperature was set to 150.degree. C., and the
dispersion medium was removed by evaporation from the conductive
adhesive liquid layers, thereby obtaining a long polarizable
electrode. The laminated sheet was passed through the drying
machine at such a speed that every portion of the laminated sheet
stayed within the drying machine for three minutes.
[0037] This long laminated sheet was press cut to form a
rectangular-shaped polarizable electrode with its carbon electrode
portion measuring 10 cm square and its lead portion (the portion
where the current collector is not covered with the polarizable
electrode) measuring 2.times.10 cm. Two such polarizable electrodes
were set up as a positive electrode and a negative electrode,
respectively, and a 80-.mu.m thick hydrophilized ePTFE sheet
("BSP0708070-2" manufactured by Japan Gore-Tex Inc.) was inserted
as a separator between them to produce a single cell. Next, the
single cell was vacuum dried at 230.degree. C. for 24 hours, after
which the cell was wrapped in an aluminum pack inside a glove box
where a dew point of -60.degree. C. or less was maintained in an
argon atmosphere. The aluminum pack used was made by cutting a dry
laminated product "PET12/A120/PET12/CPP30" manufactured by Showa
Denko Packaging Co., Ltd. and formed into a 25.times.20-cm bag-like
shape by heat sealing (one of the shorter sides was left open, and
the other three sides were heat sealed). To impregnate the
electrodes with an electrolytic solution, a propylene carbonate
solution containing 1.8 mol/L of triethylmethylammonium
tetrafluoroborate was injected as the electrolytic solution into
the aluminum pack at a reduced pressure of -0.05 MPa, and the
electrodes were left immersed in the solution for 10 minutes.
Finally, the open end of the aluminum pack was sealed by heating,
thus producing a rectangular-shaped electric double layer
capacitor. The expected operating voltage of this capacitor was 3.3
V.
[0038] The rectangular-shaped electric double layer capacitor was
stored at 25.degree. C. for 24 hours. Then, a compression pressure
(surface compression pressure) of 2.45.times.10.sup.6 Pa was
applied from both sides of the capacitor and, in this condition,
initial charging was performed by charging the capacitor up to 4.0
V at a current density of 5 mA/cm.sup.2 in a thermostatic chamber
maintained at 40.degree. C., and thereafter, the capacitor was
discharged to 0 V with the same current density. This capacitor is
designated as the capacitor of Example 1.
Examples 2 to 5
[0039] Each capacitor was produced in the same manner as in Example
1, except that the storage temperature was set to 40.degree. C.,
50.degree. C., 60.degree. C., and 70.degree. C., respectively.
Example 6
[0040] A capacitor was produced in the same manner as in Example 1,
except that the storage time was set to 12 hours.
Examples 7 to 9
[0041] Each capacitor was produced in the same manner as in Example
1, except that the surface compression pressure was set to
4.9.times.10.sup.5 Pa, 1.5.times.10.sup.6 Pa, and
4.9.times.10.sup.6 Pa, respectively.
Examples 10 to 12
[0042] Each capacitor was produced in the same manner as in Example
1, except that the temperature of the thermostatic chamber during
the charging was maintained at 50.degree. C., 60.degree. C., and
70.degree. C., respectively.
Examples 13 to 15
[0043] Each capacitor was produced in the same manner as in Example
1, except that the end-of-charge voltage was set to 3.7 V, 4.1 V,
and 4.3 V, respectively.
Comparative Example 1
[0044] A capacitor was produced in the same manner as in Example 1,
except that the rectangular-shaped electric double layer capacitor
was subjected to initial charging immediately after the production
by omitting the storing step.
Comparative Examples 2 and 3
[0045] Each capacitor was produced in the same manner as in Example
1, except that the storage time was set to 5 hours and 10 hours,
respectively.
Comparative Example 4
[0046] A capacitor was produced in the same manner as in Example 1,
except that the temperature of the thermostatic chamber during the
charging was maintained at room temperature (25.degree. C.).
Comparative Example 5
[0047] A capacitor was produced in the same manner as in Example 1,
except that the surface compression pressure was set to
9.8.times.10.sup.4 Pa.
Comparative Example 6
[0048] A capacitor was produced in the same manner as in Example 1,
except that the end-of-charge voltage was set to 4.5 V.
[0049] The manufacturing conditions for the respective capacitors
are summarized in Table 1 below.
TABLE-US-00001 TABLE 1 Surface Temperature End-of- End-of- Storage
Storage compression during charge charge temperature time pressure
charging voltage voltage (.degree. C.) (h) (Pa) (.degree. C.) (V)
(%) Example 1 25 24 2.5 .times. 10.sup.6 40 4.0 121 Example 2 40 24
2.5 .times. 10.sup.6 40 4.0 121 Example 3 50 24 2.5 .times.
10.sup.6 40 4.0 121 Example 4 60 24 2.5 .times. 10.sup.6 40 4.0 121
Example 5 70 24 2.5 .times. 10.sup.6 40 4.0 121 Example 6 25 12 2.5
.times. 10.sup.6 40 4.0 121 Example 7 40 24 4.9 .times. 10.sup.5 40
4.0 121 Example 8 40 24 1.5 .times. 10.sup.6 40 4.0 121 Example 9
40 24 4.9 .times. 10.sup.6 40 4.0 121 Example 10 40 24 2.5 .times.
10.sup.6 50 4.0 121 Example 11 40 24 2.5 .times. 10.sup.6 60 4.0
121 Example 12 40 24 2.5 .times. 10.sup.6 70 4.0 121 Example 13 40
24 2.5 .times. 10.sup.6 40 3.7 112 Example 14 40 24 2.5 .times.
10.sup.6 40 4.1 124 Example 15 40 24 2.5 .times. 10.sup.6 40 4.3
130 Comparative -- 0 2.5 .times. 10.sup.6 40 4.0 121 example 1
Comparative 25 5 2.5 .times. 10.sup.6 40 4.0 121 example 2
Comparative 25 10 2.5 .times. 10.sup.6 40 4.0 121 example 3
Comparative 25 24 2.5 .times. 10.sup.6 25 4.0 121 example 4
Comparative 40 24 9.8 .times. 10.sup.4 40 4.0 121 example 5
Comparative 25 24 2.5 .times. 10.sup.6 40 4.5 136 example 6
[0050] Expansion ratio, volumetric capacitance density, and
internal resistance were measured on each of the electric double
layer capacitors of Examples 1 to 15 and Comparative examples 1 to
6. The measurement conditions were as shown below.
(Expansion Ratio)
[0051] The expansion ratio was calculated by measuring the
thickness of the electrode portion after the fifth cycle of
charging performed under the charge/discharge conditions given
hereinafter (the thickness was measured after removing the surface
compression pressure) and by dividing the thus measured thickness
by its initial thickness of 200 .mu.m.
(Volumetric Capacitance Density)
[0052] Measuring instrument: "CDT510-4" manufactured by Power
Systems Co., Ltd.
[0053] Charging: 5 mA/cm.sup.2, 3.3 V, 3600 seconds
[0054] Discharging: 5 mA/cm.sup.2, 0 V
[0055] The capacitance at the end of the fifth cycle was obtained
by an energy conversion method, and the volumetric capacitance
density was calculated by dividing the obtained value by the volume
of the positive and negative carbon electrode portions, excluding
the current collectors, before and after the expansion. For the
calculation, analysis software ("CDT Utility" produced by Power
Systems Co., Ltd.) was used.
(DC Internal Resistance)
[0056] When measuring the volumetric capacitance density, the
internal resistance was calculated by the equation V=IR. For the
calculation, analysis software ("CDT Utility" produced by Power
Systems Co., Ltd.) was used.
TABLE-US-00002 TABLE 2 Volumetric capacitance Expansion density
(F/cm.sup.3) Internal ratio Before After resistance (%) expansion
expansion (m.OMEGA.) Example 1 130 40 31 11 Example 2 110 40 36 10
Example 3 115 40 35 10 Example 4 115 40 35 9 Example 5 115 40 35 9
Example 6 140 40 29 12 Example 7 130 42 32 12 Example 8 120 40 33
11 Example 9 100 36 36 13 Example 10 115 40 35 11 Example 11 115 40
35 11 Example 12 120 38 32 11 Example 13 105 33 32 10 Example 14
125 42 34 11 Example 15 135 45 33 12 Comparative 200 40 20 15
example 1 Comparative 180 38 21 15 example 2 Comparative 180 40 22
15 example 3 Comparative 190 40 21 15 example 4 Comparative 200 40
20 15 example 5 Comparative 300 45 15 20 example 6
[0057] As can be seen from Table 2, in the capacitor of each
example according to the present invention, the expansion ratio was
suppressed compared with the capacitor of any comparative example,
and as a result, the volumetric capacitance density significantly
increased. Comparing Example 1 and comparative example 1, it can be
seen that when the rectangular-shaped electric double layer
capacitor was stored for 24 hours, the volumetric capacitance
density increased by at least 50%. In particular, a comparison
between Example 6 and comparative examples 2 and 3 shows that when
the capacitor was stored at room temperature (25.degree. C.), if
the storage time was not longer than 10 hours in other words, the
period of time that could elapse from the moment that the
electrolytic solution was injected until the start of the charging
process, even when the capacitor was not specifically intended to
be stored, the capacitance density increased by only 10% compared
with the case where the storing step was omitted (comparative
example 1), but that when the capacitor was stored for 12 hours,
the capacitance density increased substantially (45%) compared with
the case where the storing step was omitted. A comparison between
Example 1 and comparative example 4 shows that when the temperature
during the initial charging was maintained at a temperature
(40.degree. C.) higher than room temperature (25.degree. C.), the
capacitance density increased by about 50%. Further, a comparison
between Example 2 and comparative example 5 shows that when the
pressure necessary to suppress the expansion during the initial
charging was applied to the electrodes at the start of the
charging, and when the capacitor was charged in this condition, the
capacitance density increased by at least 80%. The comparative
example 6 shows that since the end-of-charge voltage was too high
(it exceeded 135% of the expected operating voltage of 3.3 V), the
surface compression pressure was unable to resist the expansion
pressure. On the other hand, from a comparison between Example 1
and Examples 2 to 5, it can be seen that when the capacitor was
stored at a temperature (40 to 70.degree. C.) higher than room
temperature (25.degree. C.), the expansion due to the charging was
further suppressed, and the capacitance density further
increased.
INDUSTRIAL APPLICABILITY
[0058] According to the present invention, polarizable electrodes
containing specifically a carbon material having graphite-like
microcrystalline carbon are first stored for a predetermined period
of time prior to initial charging, and then the charging is
performed at a temperature higher than room temperature. Since this
serves to suppress the expansion of the electrodes, the capacitance
density of the electric double layer capacitor constructed using
these polarizable electrodes significantly increases.
* * * * *